It is standard practice in the production of metallic packaging containers and parts thereof, hereinafter generically referred to as cans, to apply a coating of an anticorrosion agent, hereinafter referred to as lacquer, to the can before it is completely assembled. Thus, for example, the sheet-metal rectangle that is eventually formed into a cylindrical tube and joined to two disks to form a three-part can is imprinted with product information and so on while it is still planar. trying to accurately print and/or coat a finished can is relatively difficult, especially when the coating job must be fairly detailed and complex as is usually the case.
During the subsequent working operations the thus coated can part is often cut, scratched, soldered, or deformed so that some of the coat is locally damaged or removed, or so that raw metallic edges are left. In addition it is not unknown to leave some parts uncoated right from the start as the coating would inhibit later operations, such as soldering. When the can is made from steel it is, however, essential that all exposed metal be painted in order to prevent corrosion.
As a result, in order to avoid damaging the overall coating of the cans during subsequent deformation or treatment, lacquers are used which are relatively soft and therefore are quite stretchable as well as mechanically fairly rugged. Such lacquers are however relatively expensive and are difficult to make due to the particular requirements. According to the type of deformation or of working which the already lacquered cans are subjected to, lacquers of different composition are necessary. Even when such lacquers are used, corrosion-prone locations are always left when the cans are subjected to cutting or stamping after their coating. This produces raw edges which can never be covered by the originally applied coating on the cans.
In order to avoid the above-mentioned disadvantages and difficulties processes are known to recoat the areas of that were damaged or where the lacquer was removed in the treatment processes. In order to apply this second coating of lacquer it is known to subject the cans to an electro-dip coating wherein at least the corrosion-prone regions of the cans are moved through the bath. In another process the corrosion-prone regions are electrostatically coated with powder and corrosion protection is obtained by melting the powder at these locations.
These two known methods are relatively expensive and necessitate quite a bit of equipment to carry them out, which, in particular when the localized damage is relatively limited as for example at the edges of cuts or stampings, is uneconomic. In addition it is necessary with these methods to make up separate batches of lacquer for the overall painting and for the subsequent coating of the damaged locations.